1. Field of Invention
This invention is in the field of radar signal processing for the measurement of target (scatterer) elevation and the acquisition and creation of 3-dimensional maps using Synthetic Aperture Radar (SAR).
2. Description of the Related Art
Synthetic Aperture Radar (SAR) radar is used for ground mapping as well as target identification. The general principle behind SAR is to coherently combine the amplitude and phase information of radar returns from a plurality of sequentially transmitted pulses from a relatively small antenna on a moving platform.
Synthetic aperture radar (SAR) systems have been developed to acquire images of stationary objects by coherently integrating phase history from multiple pulse returns. High resolution maps are achieved by coherently combining return signals reflected from transmitted pulses in the cross range direction. Formation of focused SAR images or maps requires accurate information on platform position and velocity to coherently combine pulse returns from multiple pulses. The process of aligning pulses for coherent combination is referred to as motion compensation, and is usually performed with the raw radar data, at the early stage of image formation process.
The plurality of returns generated by the transmitted pulses along a known path of the platform make up an array length. During the array length, amplitude as well as phase information returned from each of the pulses, for each of many range bins, is preserved. The SAR image is formed from the coherent combination of the amplitude and phase of return(s) within each range bin, motion compensated for spatial displacement of the moving platform during the acquisition of the returns for the duration of the array length.
The plurality of pulses transmitted during an SAR array length, when coherently combined and processed, result in image quality comparable to a longer antenna, corresponding approximately to the xe2x80x9clengthxe2x80x9d traveled by the antenna during the array length.
Using a SAR, it is desired to map not only the range, azimuth, r,xcex8, position of particular scatterer but also the height of the scatterer, z. Typically, this has been done using Interferometric Synthetic Aperture Radar (IFSAR), using a plurality of antennas located on the same moving platform. Each antenna acquires its own set of data, then the sets of data are used in interferometer fashion to resolve z for each scatterer. The phase of the returns from each antenna needs to be preserved accurately to extract the mapping information. Such a radar is discussed by Graham, L. C. in Synthetic Interferometer Radar for Topographic Mapping, Proceedings of the IEEE, Vol 62, No 6, pp 763-768, June 1974, incorporated herein by reference in its entirety.
In the alternative, the same interferometer information can be obtained from multiple passes using a single SAR antenna. The motion compensation of the acquired data has to be done to a high degree of accuracy for coherent processing and subsequent interferometric comparison. The radar returns acquired during multiple passes have to be stored for subsequent use and motion compensated to fractions of a radar frequency wavelength so as to preserve the phase information contained therein. Such phase related accuracy can prove costly and elusive, especially if the subject target (scatterer) has moved between each of the multiple passes.
In either case, the complexity of obtaining elevation, 3D information about target (scatterers) is reflected in cost constraints related to gathering amplitude and phase information creating a second map to use in an interference process with the information from a first map.
Above limitations are avoided by a method for measuring the height of a radar target located at an actual height above a horizontal plane at a location within the horizontal plane using a synthetic aperture radar. The synthetic aperture radar is mounted on a moving platform. The moving platform moves along a continuous climbing path with respect to the horizontal plane. The steps for measuring target height are:
acquiring a first synthetic aperture map containing the target from a first altitude above said plane along said path;
acquiring a second synthetic aperture map containing the target from a second altitude above said plane along said path;
acquiring a third synthetic aperture map containing the target from a third altitude above said plane along said path;
performing a monopulse measurement from information contained in said first synthetic aperture map, said second synthetic aperture map, and said third synthetic aperture map to extract a first height of the target;
comparing said first synthetic aperture map and said second synthetic aperture map to extract a second height of said target using a first interferometric comparison;
comparing said second synthetic aperture map and said third synthetic aperture map to extract a third height of said target using a second interferometric comparison;
analyzing said first synthetic aperture map, said second synthetic aperture map and said third synthetic aperture map using shadow analysis to extract a fourth height of said target;
computing a fine height from change in phase at said location by comparing said first synthetic aperture map with said second synthetic aperture map and said third synthetic aperture map;
fusing said first height, said second height, said third height, said fourth height and said fine height to calculate said actual height of said target above said plane.